値の setget が property() で設定したカスタム関数で
行われるように manage された属性値　だと思います

def getAgeSabaYomi(aFem):
return int(aFem.trueAge * 0.9)

def setAge(aFem, age):
if age < 20:
raise AttributeError, "Saba Yomosugidesu!"
aFem.trueAge = age

class Girl(object):
def __init__(self):
self.trueAge = 20
age = property(getAgeSabaYomi, setAge)


>>> g = Girl()
>>> print g
<__main__.Girl object at 0x00810500>
>>> print g.age
18
>>> g.age = 30
>>> print g.age
27
>>> g.age = 17
Traceback (most recent call last):
File "<stdin>", line 23, in ?
File "<stdin>", line 6, in setAge
AttributeError: Saba Yomosugidesu!